The present invention relates generally to synchronous rectifiers and DCxe2x80x94DC converters. More particularly, it relates to a method and circuit for reducing or eliminating body diode current and the resultant energy losses in synchronous rectifiers having electronic switches (e.g. MOSFETs) with integral body diodes.
Synchronous rectifiers (including synchronous DCxe2x80x94DC converters) are widely used in computing and telecommunications. They often provide low voltage, high current power required to operate microprocessors and other high speed electronics. Synchronous rectifiers employ several electronic switches (typically low on-resistance MOSFETs) to control current flow through a transformer and rectifying circuit. MOSFETs have an integral body diode that can conduct when the transistor is turned off. In a synchronous rectifier, the body-diodes of the rectifier MOSFETs will each conduct for a short portion of each switching cycle. Body-diode conduction in the MOSFET is a source of substantial energy loss and inefficiency because the diode has a rather large forward voltage drop (typically about 1.0 volts), and because of reverse recovery of the diode which occurs when the body diode conduction ceases. In low voltage applications (e.g. 1.2 volt output power supplies) the forward voltage drop across a body diode can produce exceptionally large energy loss.
In order to provide synchronous rectifiers with small size and weight, low cost, and rapid load variation capability (an important consideration for powering microprocessors), it is best to operate synchronous rectifiers at high frequency. However, body diode conduction losses and associated reverse recovery losses increase dramatically with increasing frequency, resulting in low conversion efficiency. Therefore, in order to increase the operating frequency of synchronous rectifiers, techniques must be found to decrease or eliminate body diode conduction.
Body diode conduction losses can be reduced by shortening the dead time between switching cycles. However, this increases the performance requirements of the gate driver circuit, and does nothing to ameliorate reverse recovery losses. Also, reducing the dead time cannot eliminate body diode conduction, since a nonzero dead time is essential when operating a synchronous rectifier.
It would be an advance in the art of synchronous rectifier design to provide a synchronous rectifier with reduced or eliminated body diode conduction losses and no reverse recovery losses. It would be particularly useful to provide a method of reducing or eliminating body diode conduction in synchronous rectifiers applicable to many different kinds of synchronous rectifiers, such as current-doubler synchronous rectifiers, center-tapped synchronous rectifiers and full-wave synchronous rectifiers.
The present invention includes method for reducing body diode conduction in a synchronous rectifier. The synchronous rectifier has a secondary circuit with a rectifier switch with an integral body diode. The method includes the step of producing a steering current in the secondary circuit before a first dead time. The steering current opposes flow of freewheeling current through the integral body diode during the first dead time. In this way, the present invention can reduce or prevent entirely the body diode conduction during the first dead time. The steering current can be produced before or during the first dead time.
A turn-off pulse can be applied to the secondary after a power pulse.
The synchronous rectifier will typically have a transformer with a primary winding and a secondary winding; the secondary winding will be connected to the secondary circuit. A body diode current during a second dead time can be reduced or eliminated by short-circuiting the primary winding. The present invention is applicable to many kinds of synchronous rectifiers, including center-tapped synchronous rectifiers, current doubling synchronous rectifiers, and full bridge synchronous rectifiers.
The present invention also includes a method for reducing or eliminating body diode conduction in a synchronous rectifier during a second dead time. The synchronous rectifier has a transformer with a primary winding and a secondary winding. The method includes the step of short circuiting the primary winding during a second dead time. Short-circuiting the primary winding prevents the body diode conduction during the second dead time, and can be performed in combination with the steering current, or can be performed separately.
The present invention also includes a synchronous rectifier comprising a secondary circuit with a rectifier switch, an output inductor connected to the rectifier switch, and a circuit for producing a steering current in the secondary current. The steering current opposes freewheeling current from the output inductor during a first dead time, thereby preventing body diode conduction. The synchronous rectifier can also have a circuit for short-circuiting the transformer primary winding. The circuit for producing the steering current can be a multi-resonant primary circuit, a full bridge primary circuit, or a pulse generator coupled to the secondary circuit. The secondary circuit can be a center-tapped synchronous rectifier, current doubling synchronous rectifier, and full bridge synchronous rectifier.
The present invention also includes a synchronous rectifier with a transformer with a primary winding and a secondary winding, a primary circuit connected to the primary winding, a secondary circuit connected to the secondary winding, the secondary circuit having a rectifier switch with an integral body diode and an output inductor connected to the switch. The synchronous rectifier also has a circuit for producing steering current in the secondary circuit during a first dead time. The steering current opposes flow of freewheeling current from the output inductor during the first dead time.
The present invention also includes a synchronous rectifier full bridge primary circuit having four switches connected in a full bridge topology. The transformer primary is connected across the full bridge. The full bridge topology can provide the steering current and primary winding short circuit for preventing body diode current during both first and second dead times. The secondary circuit coupled to the primary full bridge circuit can be a center-tapped synchronous rectifier, current doubling synchronous rectifier, or full bridge synchronous rectifier, for example.